In an age of nationalisation of international educational (leadership) policy, or vice versa, the politics and conflicts behind such policies often take centre stage to the detriment of (research-based) professional expertise. In response, this article develops a nuanced critical realism to propose a practice-based development and implementation of educational (leadership) policy reforms. Based on empirical reports of head teachers' subversive practice, the article concludes by highlighting that professional expertise is a central component, dubbed 'formless capability' (potency), that all stakeholders use to turn policy intentions into policy actions (Actuality). Actuality consists of the three traditional domains of critical realism -- the real, the actual and the empirical -- whose importance is derived from whether it is a means to (incomplete action) or an end in itself (complete action). It is argued here that empirical experiences of leading schools are the stage of actuality that is directly linked to the complete action of students' learning and, therefore, stands the most chance of being faithfully and successfully embedded and implemented. Policy mechanisms, therefore, need to reflect such research-based knowledge.